The 3rd-Generation Partnership Project (3GPP) is continuing development of the fourth-generation wireless network technologies known as Long-Term Evolution (LTE). Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-10, and further improvements are being discussed in the context of new features for Release-11. In heterogeneous network deployments, a mixture of network nodes of different maximum transmit powers and overlapping coverage areas of different sizes are deployed. The term network node here can refer to, for example, a base station, an access point, or a relay node. In some contexts, the more general terms “point” or “transmission point” are used.
One example of such a deployment is seen in the system 100 illustrated in FIG. 1, where several lower-power pico-nodes 120, each with a respective coverage area 150, are deployed within the larger coverage area 140 of a higher power macro-node 110. The system 100 of FIG. 1 is suggestive of a wide-area wireless network deployment. However, other examples of low-power nodes in heterogeneous networks are home base stations, also referred to as femtonodes, and relays.
Throughout this document, nodes in a network are often referred to as being of a certain type, e.g., a “macro-” node, or a “pico-” node. However, unless explicitly stated otherwise, this should not be interpreted as an absolute quantification of the role of the node or point in the network but rather as a convenient way of discussing the roles of different nodes or points relative to one another. For example, this usage simply indicates that one point, e.g., a macro-node, has a relatively high maximum transmit power and thus a relatively large coverage area, while another point, e.g., a pico-node, has lower transmit power and a smaller coverage area. Thus, a discussion about macro- and pico-nodes could just as well be applicable to the interaction between micro-nodes and femto-nodes, for example. It should also be noted that in a heterogeneous deployment consisting of macro-nodes and pico-nodes, the communication links associated with the macro-nodes and the communication links associated with the pico-nodes are often referred to as belonging to different layers, e.g., macro layer and pico layer, respectively.
One aim of deploying low-power nodes such as pico-nodes within the coverage area of a macro-node is to improve system capacity, by means of area-splitting gains. In addition to improving overall system capacity, this approach also allows users to be provided with a wide-area experience of very-high-speed data access, throughout the network. Heterogeneous deployments are in particular effective to cover traffic hotspots, i.e., small geographical areas with high user densities. These areas can be served by pico-nodes, for example, as an alternative deployment to a denser layer of macro-nodes.
One way to operate heterogeneous networks is to apply frequency separation between the different layers. For instance, the macro-node 110 and pico-nodes 120 pictured in FIG. 1 can be configured to operate on different, non-overlapping carrier frequencies, thus avoiding any interference between the layers. Another approach to operating a heterogeneous network is to share radio resources between layers. Thus, two (or more) layers can use the same carrier frequencies, by coordinating transmissions across macro-nodes and under-laid pico nodes. In the following discussion it is assumed that the macro-nodes and pico-nodes share a common set of frequencies.
Thus, as can be seen in FIG. 1, in a heterogeneous cell deployment a high-power point and one or more low-power points both transmit signals to mobile stations. Coverage areas corresponding to the low-power points fall at least partly within the coverage area for the high-power point, so that mobile stations within range of a low-power point are also within range of the high-power point.
In one approach to heterogenous cellular-network deployments, the pico-nodes create separate cells with separate cell identities that are different from the cell created by the macro node. In this case, the pico nodes must generally transmit a full complement of common signals and control channels. In an LTE context, this may include, for example, the cell-specific reference signals (CRS), primary synchronization signal (PSS) and secondary synchronization signal (SSS) and the broadcast channel (BCH).
An alternative approach to heterogeneous cellular-network deployment is one in which the pico-node does not correspond to a cell of its own. In other words, a pico-node in these deployments does not have a cell-ID distinct from that provided by the macro-node, but instead simply provides a data-rate and capacity “extension” of the overlaid macro-cell. In an LTE environment, for example, CRS, PSS, and SSS, as well as channels that rely on CRS for reception, such as PDCCH and PBCH, are then transmitted from the macro-node. The data-carrying PDSCH for a given mobile station, on the other hand, can be transmitted from the pico-node. So called UE-specific reference signals are then also transmitted from the pico-node, together with the PDSCH, for aiding in PDSCH demodulation and detection.